Switched Mode Power Supply Overshoot problem

Guest

Hi, all-
I hope someone could offer me some advice.
My design has a buck switched mode power supply (12v to 1.8v,
MAX1993).
On startup, there's an overshoot on the voltage, and the contoller
shuts down on
the over-voltage protection.
What could be the causes? maybe wrong values on the output inductor,
or capacitor?
maybe wrong ESR value on the cap? maybe layout design problem?
Thanks for any help, guys,
Harpoon

Hi, all-
I hope someone could offer me some advice.
My design has a buck switched mode power supply (12v to 1.8v,
MAX1993).
On startup, there's an overshoot on the voltage, and the contoller
shuts down on
the over-voltage protection.
What could be the causes? maybe wrong values on the output inductor,
or capacitor?
maybe wrong ESR value on the cap? maybe layout design problem?
Thanks for any help, guys,
Harpoon

Click to expand...

A copy of the schematic would be very helpful. But
increasing the capacitance of the output filter may help.
The best cure is probably in the feedback divider and filter
section of the circuit.

Hi, all-
I hope someone could offer me some advice.
My design has a buck switched mode power supply (12v to 1.8v,
MAX1993).
On startup, there's an overshoot on the voltage, and the contoller
shuts down on
the over-voltage protection.
What could be the causes? maybe wrong values on the output inductor,
or capacitor?
maybe wrong ESR value on the cap? maybe layout design problem?
Thanks for any help, guys,
Harpoon

Click to expand...

Good luck with that one. It is a highly complex, read I've never really
looked very hard at that stuff much before, controller. Working out what it
thinks its transfer function might be could be hard. It's probably some
wierd thing that is not suited to 'normal' linear analysis.

The dirty sheet does mention possible instabilities due to the capacitor ESR
zero being too high. Voltage feedback is a simple divider and there is no
feedback available to tailor the response of the internal error amplifier if
there is one in there.

That might suggest that the overall voltage loop including the capacitor is
second order up to the ESR zero where it becomes first order and crosses
over at some higher frequency. If the ESR zero is too high then is crosses
over second order and is unstable...... BTW this is speculation.

Anyway, one possibility is that with such a large VIN/VOUT differential you
can rapidly ramp up your filter inductor current to some excessive level and
when the output voltage reaches the set level the controller switches things
off BUT you have still put energy into the inductor and that gets delivered
to the output causing the overvoltage.

Cures for that would be reducing the absolute current limit level and/or
increasing the inductor value so it doesn't store too much excess energy.

If I'm right about the loop behaviour (pigs might fly) then you might split
the feedback resistor from the output node into two in series with a ten to
one ratio and add a capacitor across the larger resistor..... and then
experiment with the value of the capacitor. That has two effects.

The first is it introduces a low frequency zero at the input making the loop
first order up to the frequency where the smaller resistor cancels that
zero. This is good because is makes the loop less conditionally stable.

The second is that such a response results in a more damped behaviour and,
in particular, a less snappy start up. Rather than starting up balls to the
wall it should bring the output voltage up 'gradually'.

Further than that I wouldn't like to guess. All of the above is speculation
based on 'normal' controllers and should but might not apply to this one.

Hi, all-
I hope someone could offer me some advice.
My design has a buck switched mode power supply (12v to 1.8v,
MAX1993).
On startup, there's an overshoot on the voltage, and the contoller
shuts down on
the over-voltage protection.
What could be the causes? maybe wrong values on the output inductor,
or capacitor?
maybe wrong ESR value on the cap? maybe layout design problem?
Thanks for any help, guys,

Click to expand...

Certainly is a convoluted architecture.

This thing claims to have a built-in slow start. With a constant
on-time topology, this is frequency modulated. Are you starting up by
using the enable line, or just whanging the input with 12V?

Without a well established 5V bias supply, you can't count on the slow
start working. This is also the supply that is needed to switch from
2V5 to 1V8 regulation.

At lower output voltage settings, the app circuit has the output
terminal directly returned to the FB input, preventing the
conventional use of output feed forward.

Guest

Hi all,
Thanks for all the help!
Just wanted to inform you that the problem was solved, with help from
MAXIM support:
The current-limit setting in my design was wrong: I supply 4A max, and
my limit was
set to 14A, so the initial current was too much for the controller.
the fix was to set the current limit threshold via divider resisors,
and change the low
side mosfet to one with higher Rds resistance, as per the datasheet.
Thats all,
thanks again for all the help!
Harp.

Want to reply to this thread or ask your own question?You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.